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CO2 in the air dissolves into rain water, making them slightly acidic. Is that enough to deplete the surrounding air out of CO2? If yes, how fast does this occur? Is it only with the first drops of rain or do you need several hours?

How long does it take for the surrounding atmosphere to regain its original CO2 contents, assuming this actually happens?

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    $\begingroup$ I would have thought that the CO2 would be dissolved as the cloud formed, rather than as it rained, and hence there would be plenty of time for the mixing of the atmosphere to even out any changes in concentration, but that is just my intuition. Interesting question. $\endgroup$
    – Dikran Marsupial
    Nov 17, 2015 at 9:59
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    $\begingroup$ the dimensionless Henrey constant is something about 0,83 so you have 0,83 x 400 ppm = 332 ppm CO2 in the water. give nthat the molar concentration in water and air is almost the same but the air column is very very high compared to the liquid water volume I'd say the effect on the athmosphere is small to rediculosly small. $\endgroup$
    – mart
    Nov 17, 2015 at 10:51
  • $\begingroup$ @mart nice estimate. I found two websites that suggest the number 355 PPM. Here ehow.com/info_12300030_much-co2-rain-water.html and here: chemistry.wustl.edu/~edudev/LabTutorials/Water/FreshWater/… Given that water vapor is rarely more than 3% of the atmosphere and only a fraction of that can turn into rain, and the PPM is slightly lower in rain-water than air, that's a pretty slow way of pulling CO2 from the atmosphere. what's more, as rain water evaporates, that CO2 is returned to the atmosphere. $\endgroup$
    – userLTK
    Nov 17, 2015 at 23:55

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Here is the mass-calculation. We will consider a column of the atmosphere with a footprint of 1m × 1m. This column weighs about 10,000 kg (per square metre). In these days of climate change we will assume the current average CO2 concentration is 400 ppm, yielding a total mass of CO2 in this column of 4 kg. The rain doesn't wash out the entire thickness of atmosphere but only (in general) the lowermost troposphere. We will assume, generously, that some 2.5 kg of this 4 kg of CO2 is available to be scavenged by raindrops. Now consider a cloudburst of raindrops, equivalent to some 100 mm on the ground. So we have 0.1 cubic metres, or 100 kg, of fallen raindrops within our square metre column of air. The solubility of CO2 in air is strongly temperature dependent, up to 2.5 g per kg at 10 deg C. But at such a low temperature we couldn't achieve 100 mm of rainfall, so we will compromise at about 1.5 g per kg at 25 °C. So, assuming complete CO2 saturation within the raindrops (unlikely) our 100 kg of rain will contain 0.15 kg of CO2 , scavenged from about 2.5 kg of CO2 in the 'rained out' column of air, or about 6% of the low atmospheric CO2 .

Bear in mind that this assumes an enormous rainfall intensity, 100% CO2 saturation of the water and equilibrium chemical dynamics. After the raindrops hit the ground at least half of it will immediately re-evaporate back into the air, leaving, at absolute most, about 3% of the atmospheric CO2 leached out of the atmosphere that will be available to react with the soil, rock or biosphere. Also consider that this is but one of several important processes affecting CO2 transience, such as photosynthesis, respiration, volcanism, industrial pollution, etc. So the CO2 estimates that you read about are average values. Advection and turbulent air mixing should ensure that the CO2 regains approximately normal concentration within an hour or two after rainfall.

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  • $\begingroup$ The 'hour or two' is not pure speculation, it is a rough estimate based upon a typical duration of rain-storm and footprint size, and a typical advection rate of replacement air. There are so many variables that it is pointless to try to be more precise. The back of an envelope calculation is not meant to be precise, but is sufficient to demonstrate that rain cannot greatly affect the CO2, even under extreme conditions. $\endgroup$
    – Gordon Stanger
    Nov 17, 2015 at 20:00
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    $\begingroup$ There are estimates posted above that CO2 by PPM in a rain-drop is about 355, so 1 KG of rain using 355 PPM estimate, is about .355 grams (equivalent) and when we adjust for molecular weight, about .87 grams CO2 per KG of rain. Your estimate, 1.5 g per KG isn't too far off, but it's nearly twice the actual amount. Something else to consider, especially in warmer temperatures, a share of rain water evaporates so a share of that CO2 is fairly quickly returned to the atmosphere. Props for working out the calculation though. $\endgroup$
    – userLTK
    Nov 18, 2015 at 0:08
  • $\begingroup$ ppm is not mass% $\endgroup$
    – mart
    Nov 18, 2015 at 7:32
  • $\begingroup$ @mart I did that. That's why it went from .355 grams (equivalent) to .87 grams (real). CO2 being about 2.4 times the mass of H20. $\endgroup$
    – userLTK
    Nov 19, 2015 at 3:06
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    $\begingroup$ 355, 400 ppm? Who gives a stuff? - it's just a back of an envelope calculation to demonstrate that the scale of CO2 scavenging by rain is trivially small. $\endgroup$
    – Gordon Stanger
    Nov 19, 2015 at 4:00
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If it rains hard enough it will completely strip the atmosphere of CO2 in that area. A very simple experiment can chart the CO2 captured by rain droplets in relation to the intensity of the rain over time. Once it's done raining the replenishment of CO2 in the depleted area is fairly quick. It takes about 3 minutes 30 seconds for the CO2 to evolve. The evolution from start to finish it's a deceleration event. Hope this helps everyone out.

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    $\begingroup$ Can you add references? That would be great, since it contradicts other answers. $\endgroup$
    – Jan Doggen
    Oct 11, 2019 at 21:57
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    $\begingroup$ About 3 yrs. ago MIT was conducting their own experiments on this exact topic. It was to better pinpoint the absorption rate of aerosols in a droplet of water falling through the atmosphere. The Earth is like a huge wet scrubber. If a person understands how a scrubber works it's easier to imagine what is happening with rain and atmosphere. Collect the first 100 ml of rain in a rain storm and immediately put the ph meter in the water. You should see a TDS of 10 to 20 and a ph of less than 7 $\endgroup$
    – Brian
    Oct 16, 2019 at 15:33
  • $\begingroup$ What exactly that ph reading might be depends on the elevation that the droplet started at. In the first 30 seconds of that sample the ph will come down fairly fast. Collect a 100 ml of the last of the rain in the same rain storm. If it reads 7 on the ph meter than the co2 is gone. When it comes to knowing the exact numbers, no one knows. Elevation, droplet size, droplet quantity and duration all play a roll. Each rain storm can yield a slightly different result because of these factors. $\endgroup$
    – Brian
    Oct 16, 2019 at 15:51
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I appreciate all of the thoughtful responses above and for the mathematical understanding that I personally would not have about this topic. One factor that may not have been considered in this discussion is the association of CO2 and water in the atmosphere and how this may have a significant affect on increasing the CO2 integration in raindrops. Another factor that must be considered is in the presence of competing organic and inorganic matter suspended in the atmosphere. Raindrops form not as a simple threshold reached between temperature, barometric pressure and water content. We need to first differentiates free water versus bound water . In addition, we have to consider the concentration of ice nucleating Protein present on the membranes of bacteria suspended in the atmosphere. These catalysts are key in initiating the water droplet formation. Bacteria that utilize this process to return to earth are carbon fixers and also help slow evaporation. Water cycle is a part of life or better yet, it is life. Not knowing all of these specific factors and not having the mathematical knowledge I don’t think I would ever be able to figure this problem out. However, I used to live in Los Angeles. The visual effect of a rainfall transforming smog to clear blue skies and seeing the snow capped mountains once or twice a year tells me there must be something more here.

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